Straight from the Sendmail.org FAQ, seems like there is no sendmail feature supporting this:
Subject: Q4.20 -- How can I automatically copy messages based on sender or recipient addresses?
Date: June 30, 2000
Updated: February 27, 2001
Updated: June 5, 2001
It would require custom programming. You could either write a mail filter using the new (currently not supported) Milter API in sendmail 8.10 and later (see libmilter/README) or you could look at some of these other unsupported hints:
Axel Reinhold's logall.c module
Message-Id: <33AA593A.4B701C3F@syntegra.nl>
the procmailrc man page
Note that no such feature has been added to sendmail. When asked about this one of the sendmail developers said it was "because we still believe a bit in privacy." IBM Certified Specialist - MQSeries